In the practical use for the production of the α-olefins, it is highly desired to develop a novel heterogeneous catalyst system. The metal complexes immobilized into the clay interlayers show a great potential as heterogeneous catalysts due to their excellent processability. In this study, nine types of heterogeneous procatalyst Ln/Ni 2+-micas were synthesized via a one-pot preparation method, which includes both the condensation reaction of the ligand derivatives and the intercalation of the ligands into the Ni 2+ ion-exchanged fluorotetrasilicic mica interlayer. The ligand structures of the prepared procatalysts were [Ln: R-N = C(Nap)-C(Nap) = N-R] [(Nap = 1,8-naphthdiyl) (L 1, R = 2-MePh; L 2, R = 2-FPh; L 3, R = 2-BrPh; L 4, R = 4-MePh; L 5, R = 4-FPh; L6, R = 4-BrPh; L7, R = 2,4-F 2Ph; L8, R = 2,4-Br 2Ph; L9, R = 2,6-F 2Ph). At 50 ℃ and 0.7 MPaethylene pressure, the triisobutylaluminum-activated L1-L6/Ni 2+-mica showed a catalytic activity for the ethylene oligo-/polymerization in the range of 334 - 549 g-ethylene •g-cat –1 •h –1. A high catalyst activity was obtained when the substituent having a larger steric bulk than that of a methyl substituent was introduced at the ortho-position of the aryl rings. The introduction of the fluorine substituent as a strong electron-withdrawing group to the para-position also increased the catalytic activity. The L 2, L 4, L 5, and L 6/Ni 2+-micas showed moderate selectivities to oligomers consisting of C 4-C 20 in the range of 19.9 - 41.6 wt% at 50 ℃. The calculated Schulz-Flory constants α based on the mole fraction of C 12 and C 14 were within 0.61 - 0.78.
The oligomerization of ethylene is a major industrial process to produce α-ole- fins, which are used in the ethylene polymerization as a co-monomer and the preparation of a variety of economically important chemicals. Since Brookhart and coworkers found that the high electrophilicity of the cationic species of the α-diimine Ni(II) and Pd(II) complexes afforded a high activity for the polymerization of α-olefins, late transition-metal technology has attracted great attention in both academic and industrial research [
Both the high activity of the catalysts and their ability to control the molecular weight of the products are attributed to the following key features of the catalyst design. A major research direction in this field is modification of the α-diimine ligand or bis(imino)pyridine ligand structures [
Although homogeneous catalysts contain uniform and well-defined active sites, there are some drawbacks when these catalysts were used for industrial applications, especially the difficulty in separating the catalysts, products, and solvent [
Recently, we developed novel heterogeneous catalyst precursors (procatalyst) immobilizing late transition-metal complexes in clay mineral interlayers, which were prepared by the direct reaction of a cation-exchange clay mineral (host material) and α-diimine or bis(imino)pyridine ligands (guest material) [
In this study, we report the synthesis of a series of novel heterogeneous catalyst immobilizing α-diimine Ni(II) complexes with an acenaphthyl backbone via a one-pot preparation method. Their catalytic properties for the ethylene oligo-/ polymerization were also investigated.
The chemicals used for the procatalyst preparation and the ethylene oligo-/po- lymerization were purchased from Kanto Chemical Co., Inc., and Tokyo Chemical Industry Co., Ltd. The fluorotetrasilicic mica (Na+-mica) was supplied by COOP Chemical Co., Ltd. The solvent used for the procatalyst preparation and the ethylene oligo-/polymerization was degassed by N2 bubbling, and then dehydrated over 400˚C-dried molecular sieves (MS-13X) before use. The preparation and handling of the procatalysts were performed by a standard Schlenk technique under a N2 atmosphere.
As shown in
intercalation of the formed ligands into the Ni2+-mica interlayer, and the reaction of the ligand and metal ions in the interlayer to form the corresponding metal complexes.
Five grams of Na+-mica (amount of Na = 10.6 mmol) was added to a solution of nickel nitrate hexahydrate (5.45 g, 18.74 mmol) dissolved in deionized water (125 ml) using a 200-ml Erlenmeyer flask. The resulting suspension was maintained at 30˚C for 24 h. The solid part was recovered by filtration, and these consecutive manipulations were repeated. The crude product was washed five times with ethanol and dried for about two weeks at ambient temperature. The product was calcined at 200˚C for 4 h, and then dried in a vacuum at 200˚C for 4 h to obtain the Ni2+-mica. The composition of Ni2+-mica was determined by an X-ray fluorescence (XRF) analysis (PW2400; PANalytycal B.V.).
Nine types of Ln/Ni2+-mica with a series of substituted acenaphthenequinone ligands (Ln) were prepared by the one-pot preparation method with 1 equivalent of acenaphthenequinone, 2.5 equivalents of aniline derivatives, and a predetermined amount of Ni2+-mica. The reaction ratio of the ligand derivatives and Ni2+-mica was controlled in order to obtain the Ln/Ni2+-mica with the Ln amount of 0.40 mmol・g-mica−1 because the Ni2+ content of the Ni2+-mica was 0.54 mmol・g-mica−1. Our previous study concluded that acetonitrile was the best preparation solvent due to its strong polarity which is necessary to swell the Ni2+-mica [
L1/Ni2+-mica: An acetonitrile (10 ml) solution of acenaphthenequinone (21.9 mg, 0.12 mmol), o-toluidine (32.2 mg, 0.30 mmol), and Ni2+-mica (300 mg, amount of Ni = 0.16 mmol) was stirred under reflux conditions for 24 h using a Schlenk flask. After the preparation, almost all of the solvent was removed by decantation using a syringe, the solid product was washed with toluene three times, and then finally washed with hexane two or more times by the same decantation manner to remove the free ligand. The residual solvent was removed under vacuum for 4 h. The L1/Ni2+-mica was obtained as a yellow powder.
L2-L9/Ni2+-mica: Based on the above procedure, the procatalysts were prepared with acenaphthenequinone (21.9 mg, 0.12 mmol) and various aniline derivatives (0.30 mmol).
Control: The ligand L5 (bis (4-fluorophenylimino) acenaphthene) and the conventional Brookhart-type nickel dibromide complex L5-NiBr2 were used as the controls. The ligand L5 and the complex L5-NiBr2 were prepared according to the procedures described in the literature with a slight modification [
Characterization of procatalysts: The FT-IR spectra of the procatalysts were recorded by an FT-IR spectrometer in the range of 2000 - 1300 cm−1 (FT/IR 4100; JASCO Corporation). The specimen was prepared by the following method. The procatalyst was mixed with dried SiO2 as a binder (procatalyst/SiO2 = 2 by weight, total amount = 10 mg), and then molded into a 10 mm-φ wafer in a glovebox under a N2 atmosphere. The wafer was placed in a specially designed specimen holder and used for FT-IR measurement. The basal spacing of the procatalyst was determined by the X-ray diffraction (XRD) measurement (Ultima III; Rigaku Corporation) with the following operating conditions: X-ray = CuKα line (λ = 0.154 nm), scan rate = 1.0 degree・min−1, scan angle = 3 - 15 degree, voltage = 40 kV, and current = 40 mA. To avoid exposure to air during the XRD measurement, the well-mixed paste of the procatalyst and a small amount of dry liquid-paraffin were placed on a glass specimen holder and covered with a polyester film under a N2 atmosphere.
The ethylene oligo-/polymerization was conducted using a 120-ml autoclave equipped with a magnetic stirrer. n-Heptane (50 ml), n-tridecane as the internal standard (0.5 ml, 10 vol.% in toluene), the prepared procatalyst (4 mg, theoretical amount of Ni complex = 1.6 μmol), and an alkylaluminum compound as the activator (480 μmol, Al/Nicomplex = 300) were successively added to the autoclave under a N2 atmosphere. The autoclave was placed in a water bath that was maintained at the reaction temperature (50˚C or 70˚C). Ethylene was continuously supplied with the ethylene pressure maintained at 0.7 MPa. After 1.5 h, 1.0 ml of a gas sample was extracted from the gas-phase in the autoclave and analyzed by a gas chromatograph equipped with a thermal conductivity detector (GC-8A; SHIMADZU Corporation) to determine the amount of C4 products in the gas-phase. The autoclave was cooled to 0˚C in an ice bath and the produced oligomers (C6-C20) were quantitatively determined by a gas chromatograph equipped with a flame ionization detector (GC-14A; SHIMADZU Corporation). The reaction was terminated by the addition of ethanol, and the produced solid polyethylene was recovered by filtration, dried, and weighed. During the reaction, the ethylene flow into the reactor was measured by a mass flow meter connected to the ethylene supply line. The activities in all the catalytic runs were determined by the total amount of the ethylene consumption.
The physicochemical properties of the prepared Ni2+-mica were determined by an X-ray fluorescence (XRF) measurement as summarized in
The Ln/Ni2+-mica was characterized by FT-IR and X-ray diffraction (XRD) measurements.
Mica-type clay minerals are constructed by stacking of 2:1 layers of which negative charge is compensated by interlayer cations [
Clay mineral | Compositionb (wt%) | Amount of Ni2+ c | Exchange rate (%) | BET SAd (m2・g−1) | ||||
---|---|---|---|---|---|---|---|---|
Na2O | MgO | SiO2 | Fe2O3 | NiO | ||||
Na+-mica | 6.6 | 33.7 | 58.9 | <0.1 | <0.1 | - | - | 1.45 |
Ni2+-mica | 1.8 | 34.3 | 59.2 | <0.1 | 4.0 | 0.54 | 69 | 3.11 |
aThe amount of fluorine was fixed at a constant value (2.48%). bDetermined by XRF. cAmount of exchanged Ni2+ ions = mmol・g-mica−1. dBET surface area determined by N2 adsorption.
The observation range of 2θ was determined with reference to the basal spacing of the anhydrous mica [
The procatalyst L5/Ni2+-mica was used for the ethylene oligo-/polymerization, and the parameters, such as an effective activator (co-catalyst) and Al/Ni molar ratio, were optimized due to the high catalytic activity. The ethylene oligo-/po- lymerization trials were conducted using four types of alkylaluminum compounds, such as poly(methylalumoxane) (PMAO), modified methylalumoxane (MMAO), triethylaluminum (TEA), and triisobutylaluminum (TIBA), with variations in the molar ratio of the Al and Ni complex (Al/Ni) from 300 to 900 at 50˚C under 0.7 MPa ethylene pressure.
The results are summarized in
In this study, the catalyst activities were determined by the total amount of the
Entry | L | Activator | Al/Ni | Activityb | Productsc (wt%) | Sαf (%) | |
---|---|---|---|---|---|---|---|
C4-C20d | Solide | ||||||
1 | 5 | PMAO | 300 | 725 | 32.2 | 45.8 | 87.9 |
2 | 5 | MMAO | 300 | trace | - | - | - |
3 | 5 | TEA | 300 | 326 | 19.9 | 61.3 | 77.0 |
4 | 5 | TIBA | 300 | 549 | 20.5 | 65.7 | 82.5 |
5 | 5 | TIBA | 600 | 656 | 20.5 | 57.1 | 91.7 |
6 | 5 | TIBA | 900 | 579 | 21.1 | 72.0 | 91.0 |
aReaction conditions; temperature = 50˚C, time = 1.5 h, ethylene pressure = 0.7 MPa (gauge), solvent = n-heptane 50 ml. procatalyst = 4 mg (Entry 1-4), 2 mg (Entry 5 and 6). bActivity determined by total amount of ethylene consumption = g-ethylene・g-cat−1・h−1. c“Products” represents the product shares. d“C4-C20” represents the amount of hydrocarbons having carbon numbers 4 - 22. e“Solid” indicates the amount of solid part recovered by filtration of reaction mixture. fSelectivity to linear α-olefin determined by amounts of C4-C20 products).
ethylene consumption. About 70 - 100 wt% of the consumed ethylene was converted into the C4-C20 (oligomers) and the solid (polyethylene) products. The residual ethylene (~30 wt%) was consumed during the formation of the soluble product such as polyethylene wax having a moderate chain length.
The objective in this study is to clarify the influence of the substituted ligand structure on the catalyst performance with regard to the activity, the selectivity to the α-olefins, and the branching densities of the products. The Ln/Ni2+-mica procatalysts were used for the ethylene oligomerization at the Al/Ni ratio of 300 and 0.7 MPa ethylene pressure along with varying the activator and reaction temperature. The results are summarized in
Entry | L | Subs. | Activator | Activity | Products (wt%) | α12 | Sα (%) | Bb (mol%) | |
---|---|---|---|---|---|---|---|---|---|
C4-C20 | Solid | ||||||||
7 | 1 | 2-CH3 | TEA | 366 | 1.8 | 92.3 | 0.83 | 76.4 | n.d. |
8 | 1 | 2-CH3 | TIBA | 482 | 3.8 | 93.3 | 0.82 | 64.0 | n.d. |
9 | 2 | 2-F | TEA | 294 | 37.5 | 49.4 | 0.64 | 88.4 | 1 |
10 | 2 | 2-F | TIBA | 433 | 41.6 | 42.9 | 0.61 | 88.8 | 1 |
11 | 3 | 2-Br | TEA | 363 | 1.4 | 97.2 | 0.76 | 64.1 | n.d. |
12 | 3 | 2-Br | TIBA | 376 | 2.4 | 99.5 | 0.76 | 80.6 | n.d. |
13 | 4 | 4-CH3 | TEA | 212 | 25.6 | 53.1 | 0.75 | 86.2 | 4 |
14 | 4 | 4-CH3 | TIBA | 334 | 27.6 | 54.0 | 0.71 | 87.8 | 4 |
4 | 5 | 4-F | TEA | 326 | 19.9 | 61.3 | 0.73 | 81.3 | 4 |
5 | 5 | 4-F | TIBA | 549 | 20.5 | 65.7 | 0.66 | 82.5 | 3 |
15 | 6 | 4-Br | TEA | 260 | 25.8 | 52.6 | 0.70 | 84.9 | 2 |
16 | 6 | 4-Br | TIBA | 424 | 24.1 | 63.7 | 0.73 | 92.3 | 6 |
17 | 7 | 2,4-F2 | TEA | trace | - | - | - | - | - |
18 | 7 | 2,4-F2 | TIBA | trace | - | - | - | - | - |
19 | 8 | 2,4-Br2 | TEA | trace | - | - | - | - | - |
20 | 8 | 2,4-Br2 | TIBA | trace | - | - | - | - | - |
21 | 9 | 2,6-F2 | TEA | trace | - | - | - | - | - |
22 | 9 | 2,6-F2 | TIBA | trace | - | - | - | - | - |
Ref. [ | 2,6-(CH3)2 | TEA | 170 | 0 | 100 | - | n.d. | - | |
Ref. [ | None | TEA | 96 | 12.6 | 71.7 | n.d. | 92.5 | n.d. |
aReaction conditions; temperature = 50˚C, time = 1.5 h, ethylene pressure = 0.7 MPa (gauge), solvent = n-heptane 50 ml, Al/Ni = 300, procatalyst = 4 mg. Abbreviations in the table are the same as those in
Entry | L | Subs. | Activator | Activity | Products (wt%) | α12 | Sα (%) | B (mol%) | |
---|---|---|---|---|---|---|---|---|---|
C4-C20 | Solid | ||||||||
23 | 1 | 2-CH3 | TEA | 258 | 2.4 | 81.2 | 0.98 | 54.6 | n.d. |
24 | 1 | 2-CH3 | TIBA | 240 | 5.5 | 92.8 | 0.82 | 58.2 | n.d. |
25 | 2 | 2-F | TEA | 130 | 43.7 | 35.3 | 0.69 | 80.9 | 4 |
26 | 2 | 2-F | TIBA | 136 | 52.0 | 16.2 | 0.57 | 86.1 | 3 |
27 | 3 | 2-Br | TEA | 134 | 3.9 | 88.4 | 0.65 | 66.0 | n.d. |
28 | 3 | 2-Br | TIBA | 187 | 4.1 | 89.6 | 0.82 | 72.2 | n.d. |
29 | 4 | 4-CH3 | TEA | 60 | 24.7 | 18.6 | 0.67 | 73.0 | 6 |
30 | 4 | 4-CH3 | TIBA | 79 | 41.7 | 27.1 | 0.66 | 75.9 | 4 |
31 | 5 | 4-F | TEA | 85 | 32.2 | 48.1 | 0.73 | 70.7 | 9 |
32 | 5 | 4-F | TIBA | 104 | 26.9 | 53.7 | 0.78 | 70.8 | 5 |
33 | 6 | 4-Br | TEA | 99 | 41.7 | 32.7 | 0.61 | 69.4 | 5 |
34 | 6 | 4-Br | TIBA | 156 | 39.7 | 31.1 | 0.65 | 73.6 | 6 |
aReaction conditions; temperature = 70˚C, time = 1.5 h, ethylene pressure = 0.7 MPa (gauge), solvent = n-heptane 50 ml, Al/Ni = 300, procatalyst = 4 mg. Abbreviations in the table are the same as those in
We first studied the effects of the ortho-substituents of the aryl rings on the performance of the catalyst (
Generally, the catalysts activities and the product distribution are dependent on both the steric bulk and the electronic effect. The sterically bulky ortho-subs- tituents of the aryl rings were considered to block the axial sites of the metal center. The mechanism of ethylene oligo/polymerization by the α-diimine nickel complex possessing ortho-substituents is represented in Scheme 1. During the chain propagation process, both the growing polymer chain and the coordinated ethylene monomer occupy the equatorial sites of the active nickel center. Meanwhile, the chain transfer process proceeds through the 5-coordinate transition state which undergoes the chain transfer with the incoming ethylene monomer from the axial sites of the nickel center [
Scheme 1. Mechanism for ethylene oligo-/polymerization with α-diimine nickel complex.
of a large quantity of the solid product with a small amount of oligomers. In addition, the sterically bulky substituent was considered to increase the ground- state energy of the resting-state species [
Compared with the hydrogen, the halogen substituents afforded both the steric effects and the electron-withdrawing effects on the active metal center. The introduction of the electron-withdrawing substituents increased the electrophilicity of the metal center. Recently, Zhang et al. calculated the net charge on the metal center in the bis(imino)pyridine iron complexes by the charge equilibration method [
We next examined the influence of the para-substituents of the aryl rings on the catalyst properties (
The results indicated that the catalyst activities of the catalysts with para- substituents mainly depended on the electronic effects relative to those of the steric bulk. Alt et al. reported that the bis(arylimino)pyridine iron complexes with 4-halogen-2-methyl substituents showed much higher activities than the complexes with 2,4-dimethyl substituents [
In terms of the oligomer selectivity, the L4-L6/Ni2+-mica showed a moderate selectivity to the α-olefins. The shares for the C4-C20 oligomers obtained by the L4-L6/Ni2+-mica were 19.9 - 27.6 wt% at 50˚C. Compared to the ligand having a sterically hindered ortho-substituent in the L1 and L3/Ni2+-mica, the less steric ligands in the L4-L6/Ni2+-mica was considered to affect the low suppression effect on the chain transfer process, resulting in the moderate oligomer selectivity. On the other hand, the L5/Ni2+-mica showed an oligomer selectivity lower than the L2/Ni2+-mica. This is presumably due to the prevention of the chain transfer process caused by the electron-withdrawing effect ofthe para-fluorine substituent. The L4/Ni2+-mica with the electro-donating methyl substituent showed slightly higher oligomer selectivity than the L5 and L6/Ni2+-mica. These results indicated that the para-halogen substituents led to an increase in the catalyst activity and slightly decreased the oligomer selectivity.
When the reaction temperature increased to 70˚C, the catalytic activity decreased and the oligomer selectivity increased in all the catalytic runs. For example, the oligomer share obtained by the L4/Ni2+-mica with TIBA increased from 27.6 to 41.7 wt% as the reaction temperature increased from 50˚C to 70˚C. The higher reaction temperature was considered to enhance the rotation of the aryl group in the complex, and thus, the chain transfer rate increased [
In all the runs, our procatalysts tended to exhibit a lower oligomer selectivity in comparison to the previous reports based on the homogeneous catalysts. For example, Alt et al. reported that the homogeneous α-diimine nickel catalysts with halogen substituents afforded a suitable selectivity to ethylene oligomers with a high activity [
A striking feature observed in the data is that the procatalysts with di-halogen substituents on the aryl rings did not show any significant activity (
To investigate the distribution of the oligomer, the molar fraction of each product was measured by a gas chromatograph, as shown in
Entry | L | T (˚C) | Activator | Oligomer distribution (%) | αa | ||||
---|---|---|---|---|---|---|---|---|---|
ΣC4 | ΣC6 | ΣC8 | ΣC20 | α12 | α18 | ||||
10 | 2 | 50 | TIBA | 37.1 | 23.0 | 16.0 | 0.6 | 0.61 | 0.43 |
14 | 4 | 50 | TIBA | 38.1 | 18.2 | 13.7 | 1.9 | 0.71 | 0.75 |
5 | 5 | 50 | TIBA | 35.2 | 18.7 | 15.2 | 1.1 | 0.66 | 0.46 |
4 | 5 | 50 | TEA | 25.1 | 21.2 | 17.5 | 1.0 | 0.73 | 0.37 |
32 | 5 | 70 | TIBA | 42.5 | 15.3 | 12.4 | 1.1 | 0.78 | 0.46 |
31 | 5 | 70 | TEA | 28.5 | 19.3 | 16.3 | 1.2 | 0.73 | 0.46 |
16 | 6 | 50 | TIBA | 35.6 | 15.9 | 13.8 | 2.2 | 0.73 | 0.68 |
aSchulz-Flory constant α, αn = Cn+2 (mol)/Cn (mol).
temperature was elevated from 50˚C to 70˚C, indicating that the β-hydrogen elimination reaction, which is the major chain-transfer reaction and gives an unsaturated chain end, favorably occurredat the higher reaction temperature in comparison to the chain propagation reaction.
The probability of the chain propagation can be described by the Schulz-Flory constant α [
In terms of the ligand structure, the α12 values increased from the L2, L5, L4 to L6/Ni2+-micas, which was in the reverse order for both the electron-withdrawing ability and the steric bulk of the substituents on the aryl rings. For example, the L2/Ni2+-mica containing the strong electron-withdrawing fluorine groups at the ortho-positions showed the lowest α12 values of 0.61. The highest α12 values were observed for the L6/Ni2+-mica which contains the weaker electron-withdrawing bromine groups at the para-positions. Moreover, the lower α12 values tended to afford higher yields of the oligomers. These results indicated that the introduction of fluorine groups, especially at the ortho-positions of the aryl rings, led to the increasing rate of the chain transfer reaction.
Noteworthy, in the oligomers formed by the L2 and L5/Ni2+-micas, two different values of the chain propagation probability α were observed. The L2 and L5/Ni2+-micas afforded significantly lower α18 values than the α18 values, while the α18 values obtained by the L4 and L6/Ni2+-micas showed similar α values. For example, the α18 value obtained by the L5/Ni2+-mica with TEA at 50˚C was 0.37, which was about half the value of α12. These phenomena could be interpreted by the existence of active centers having different structures [
One of the interesting characteristics of our heterogeneous procatalyst systems was the long lifetime of the active species during the reaction in the presence of TIBA. The catalyst lifetimes of the L1-L6/Ni2+-mica with activators were investigated by measuring the ethylene consumption profiles by a mass flowmeter.
(as standard temperature and pressure), and the periods for the full activation of the procatalysts required 52 - 72 minutes after supplying the ethylene.
The effects of the activator and the temperature on the profile were investigated.
We synthesized nine types of novel heterogeneous procatalyst Ln/Ni2+-micas with a series of substituted ligand structures via a one-pot preparation method. The basal spacings of the obtained L1-L9/Ni2+-mica were in the range of 1.20 - 1.30 nm, indicating the immobilization of the Ni complex into the mica interlayers. In the presence of the TEA or TIBA, the L1-L6/Ni2+-mica, which has one ortho- or para-substituent on the aryl ring, was active for the ethylene oligo-/ polymerization reaction, while the L7-L9/Ni2+-mica having two halogen substituents were inactive. The catalyst activity and oligomer selectivity of the procatalysts depended on the position, steric bulk, and electronegativity of the substituents on the aryl rings. The strong electron-withdrawing groups, the ortho- fluorine and para-fluorine substituents, increased the catalyst activity due to increased electrophilicity of the nickel center. The decreasing steric bulk of the ortho-substituent increased the selectivity to the oligomer fraction. Also, the procatalysts having one para-substituent afforded moderate yields of the oligomers.
In conclusion, our heterogeneous procatalyst system exhibited a moderate catalyst activity with a long lifetime as well as that of the conventional homogeneous system. In this system, the substituent and the backbone in the ligand structures are easily controlled by changing the structures of the aniline derivatives and the ketone derivatives, which are the reactants of the one-pot preparation method. Therefore, we can control the activity and the product selectivity by tuning the ligand structure. Our findings suggest that a complex with more free space at the axial position of the nickel center is necessary for increasing the oligomer selectivity. The less bulky structure of the ligand backbone is also considered to enhance the oligomer production.
This work was financially supported by theJSPS KAKENHI Grant Number 15K05514. We acknowledge Masa-aki Oshima for use of FT/IR 4100 spectrometer with the original equipment.
Yoshida-Hirahara, M., Fujiwara, S. and Kurokawa, H. (2017) Simple Preparation of Halogen-Substituted α-Diimine Nickel Complexes Immobilized into Clay Interlayer as Catalysts for Ethylene Oligo-/Polymerization. Modern Research in Catalysis, 6, 100-120. https://doi.org/10.4236/mrc.2017.62008